GB846679A - Ethylene production - Google Patents

Abstract

Ethylene is obtained by steam cracking a hydrocarbon boiling in the range C4-220 DEG F. to obtain a mixture of hydrocarbons and hydrogen, separating C5+ hydrocarbons from lighter materials, reacting C4 and lighter hydrocarbons with a portion of the hydrogen to selectively hydrogenate components less saturated than mono-olefines and by fractionation separating methane and lighter gases from heavier compounds and from the latter fraction ethylene and a heavier C2-C4 fraction. In a preferred embodiment the naphtha may be a virgin fraction e.g. C5-160 DEG F., C4-220 DEG F. or C6-220 DEG F. boiling range which is cracked at 35-75% conversion per pass in the presence of 1-3 mols of steam per mol of hydrocarbon in a cracking coil having an inlet temperature of 100-500 DEG F and outlet 1300-1600 DEG F. The effluent may be quenched first with water and then oil and further cooled and passed to a knockout drum where water is separated and then to a series, e.g. 3, of compressors, coolers and knockout drums from which C5+ material is collected. The combined C5+ material may be flashed to separate any C4 or lighter material which is recycled to the beginning of the compressor-cooler-separator series while the remaining C5+ material is recycled to the cracking stage. The C4 and lighter material is hydrogenated without addition of hydrogen by passing over palladiumalumina, cobalt molybdate-alumina, or palladium at 0-1,000 p.s.i.g. and 50-1100 DEG F. When using a sulphur-sensitive catalyst the gases may first be caustic washed. The hydrogenated gas is caustic washed, dried, demethanised and ethylene separated by fractionation. The C2-C4 residue is recycled to the cracking, part or all preferably being passed through a longer coil than the starting naphtha.